LORAN-C navigation apparatus

ABSTRACT

LORAN-C navigation apparatus is disclosed wherein digital circuitry and a microprocessor .[.is.]. .Iadd.are .Iaddend.used to automatically identify LORAN transmitting stations and .[.makes.]. .Iadd.make .Iaddend.standard hyberbolic navigation measurements. The equipment operator manually enters the group repetition rate into the apparatus for a LORAN-C chain covering the area within which the navigation apparatus is being operated. Initially, the apparatus searches all incoming signals .Iadd.as they are received .Iaddend.until signals from a master station are received regularly at the stored group repetition rate. The apparatus then closely determines the time of arrival of signals from the secondary stations of the selected LORAN-C chain before changing to a fine search mode in which the exact time of arrival of the secondary station signals is determined; the phase code of the received signals is checked to determine if the received signal is a ground or sky wave, and a determination is made if there is a defective secondary station blink code. The time difference of arrival measurements are then output visually to be plotted in a well known manner on a LORAN-C chart to locate the position of the craft upon which the apparatus is located.

This is a continuation of application Ser. No. 842,706, filed Oct. 17,1977, now abandoned.

FIELD OF THE INVENTION

This invention relates to navigational equipment and more particularlyto hyperbolic navigational equipment utilizing the time difference inthe propagation of radio frequency pulses from synchronized groundtransmitting stations.

BACKGROUND OF THE INVENTION

Throughout maritime history navigators have sought an accurate reliablemethod of determining their position on the surface of the earth andmany instruments such as the sextant were devised. During the secondworld war, a long range radio-navigation system, LORAN-A, was developedand was implemented under the auspices of the U.S. Coast Guard tofulfill wartime operational needs. At the end of the war there wereseventy LORAN-A transmitting stations in existence and all commercialships, having been equipped with LORAN-A receivers for wartime service,continued to use this navigational system. This navigational systemserved its purpose but shortcomings therein were overcome by a newnavigational system called LORAN-C.

Presently, there are eight LORAN-C multi-station transmitting chains inoperation .[.by 1980.].. This new navigational system will result in aneventual phase-out of the earlier LORAN-A navigational system. LORAN-Cis a pulsed low frequency (100 kilohertz), hyperbolic radio navigationsystem LORAN-C radio navigation systems employ three or moresynchronized ground stations that each transmit radio pulse .[.chains.]..Iadd.trains .Iaddend.having, at their respective start oftransmissions, a fixed time relation to each other. The first station totransmit is referred to as the master station while the other stationsare referred to as the secondary stations. The pulse .[.chains.]..Iadd.trains .Iaddend.are radiated to receiving equipment that isgenerally located on aircraft or ships whose position is to beaccurately determined. The pulse .[.chains.]. .Iadd.trains.Iaddend.transmitted by each of the master and secondary stations.[.is.]. .Iadd.comprise .Iaddend.a series of pulses, each pulse havingan exact envelope shape, each pulse .[.chain.]. .Iadd.train.Iaddend.transmitted at a constant precise repetition rate, and eachpulse separated in time from a subsequent pulse by a precise fixed timeinterval. In addition, the secondary station pulse .[.chain.]..Iadd.train .Iaddend.transmissions are delayed a sufficient amount oftime after the master station pulse train transmissions to assure thattheir time of arrival at receiving equipment anywhere within theoperational area of the particular LORAN-C system will follow receipt ofthe pulse .[.chain.]. .Iadd.train .Iaddend.from the master station.

Since the series of pulses transmitted by the master and secondarystations is in the form of pulses of electromagnetic energy which arepropagated at a constant velocity, the difference in time of arrival ofpulses from a master and a secondary station represents the differencein the length of the transmission paths from these stations to theLORAN-C receiving equipment.

.[.The focus.]. .Iadd.The locus .Iaddend.of all points of a LORAN-Cchart representing a constant difference in distance from a master and asecondary station, and indicated by a fixed time difference of arrivalof their 100 kilohertz carrier pulse .[.chains, described.]..Iadd.trains, describes .Iaddend.a hyperbola. The LORAN-C navigationsystem makes it possible for a navigator to exploit this hyperbolicrelationship and precisely determine his position using a LORAN-C chart.By using a moderately low frequency such as 100 kilohertz, which ischaracterized by low attenuation, and by measuring the time differencebetween the reception of the signals from master and secondary stations,the modern-day LORAN-C system provides .Iadd.an .Iaddend.equipmentposition location accuracy within two hundred feet and with arepeatability of within fifty feet.

The theory and operation of the LORAN-C radio navigation system isdescribed in greater detail in an article by W. P. Frantz, W. Dean, andR. L. Frank entitled "A precision Multi-Purpose Radion NavigationSystem," 1957 I.R.E. Convention Record, Part 8, page 79. The theory andoperation of the LORAN-C radio navigation system is also described in apamphlet put out by the Department of Transportation, U.S. Coast Guard,No. CG-462, dated August, 1974, and entitled "LORAN-C User Handbook".

The LORAN-C system of the type described in the aforementioned articleand pamphlet and employed at the present time, is a pulse type system,the energy of which is radiated by the master station and by eachsecondary station in the form of pulse trains which include a number ofprecisely shaped and timed bursts of radio frequency energy as priorlymentioned. All secondary stations each radiate pulse .[.chains.]..Iadd.trains .Iaddend.of eight discrete time-spaced pulses, and allmaster stations transmit the same eight discrete time-spaced pulses butalso transmit an identifying ninth pulse which is accurately spaced fromthe first eight pulses. Each pulse of the pulse .[.chains.]..Iadd.trains .Iaddend.transmitted by the master and secondary stationshas a 100 kilohertz carrier frequency, so that it may be distinguishedfrom the much higher frequency carrier used in the predecessor LORAN-Asystem.

The discrete pulses radiated by each master and each secondary LORAN-Ctransmitter are characterized by an extremely precise spacing of 1,000microseconds between adjacent pulses. Any given point on the preciselyshaped envelope of each pulse is also separated by exactly 1,000microseconds from the corresponding point of the envelope of a precedingor subsequent pulse within the eight pulse .[.chains pulses.]..Iadd.train.Iaddend.. To insure such precise time accuracy, each masterand secondary station transmitter is controlled by a cesium frequencystandard clock and the clocks of master and secondary stations aresynchronized with each other.

As mentioned previously, LORAN-C receiving equipment is utilized tomeasure the time difference of arrival of the series of pulses from amaster station and the series of pulses from a selected secondarystation, both stations being within a given LORAN-C chain. This timedifference of arrival measurement is utilized with special maps havingtime difference of arrival hyperbola information printed thereon. Thesemaps are standard LORAN-C hydrographic charts prepared by the U.S. CoastGuard and the hyperbola curves printed thereon for each secondarystation are marked with time difference of arrival information. Thus,the difference in time arrival between series of pulses received from amaster station and selected ones of the associated secondary stationsmust be accurately measured to enable the navigator to locate thehyperbola on the chart representing the time difference measured. Byusing the time difference of arrival information between a masterstation and two or more secondary stations, two or more correspondinghyperbolae can be located on the chart and their common point ofintersection accurately identifies the position of the LORAN-C receiver.It is clear that any inaccuracies in measuring time difference ofarrival of signals from master and secondary transmitting stationsresults in position determination errors.

There are other hyperbolic navigation systems in operation around theworld similar to LORAN-C, and with which my novel receiver can readilybe adapted to operate by one skilled in the art. There is a LORAN-Dsystem utilized by the military forces of the United States, as well asthe aforementioned LORAN-A system. Others are DECCA, DELRAC, OMEGA,CYTAC, GEE and the French radio WEB, all of which operate in variousportions of the radio frequency spectrum and provide varying degrees ofpositional accuracy.

LORAN-C receiving equipment presently in use is relatively large insize, heavy and requires relatively large amounts of power. In addition,present LORAN-C receivers are relatively expensive and, accordingly, arefound only on larger ships and aircraft. Due to the cost, size, weight,and power requirements of present LORAN-C receiving equipment, suchequipment is not in general use on small aircraft, fishing boats andpleasure boats. In addition, LORAN-C receiving equipment presently inuse .[.required.]. .Iadd.requires .Iaddend.anywhere from five to tenminutes to warm up and provide time difference measurement information.Further, present LORAN-C equipment is rather complex, having manycontrols, and the operator thereof usually must have some training inthe use of the equipment.

Thus, there is a need in the art for a new LORAN-C receiver that issmall, light in weight, has few controls and is therefore easy tooperate by inexperienced people, requires a small amount of electricalpower, and is relatively low in cost. Such equipment would fill theneeds of those who do not now have LORAN-C receiving equipment.

SUMMARY OF THE INVENTION

The foregoing needs of the prior art are satisfied by my novel LORAN-Creceiver. I eliminate much of the complex and costly automaticacquisition and tracking circuitry in prior art LORAN-C navigationreceivers and provide a small, light weight, inexpensive receiver usingrelatively little electrical power.

Four thumbwheel switches on my LORAN-C equipment are used by theoperator to enter the group repetition rate information for a LORAN-Cchain covering the area within which the LORAN-C equipment is beingoperated. Thus information entered via the thumbwheel switches is usedby an internal microprocessor to locate the signals from the master andsecondary stations of the chosen LORAN-C chain.

The receiver of my equipment receives all signals that appear within asmall bandwith centered upon the 100 Khz. operating frequency of theLORAN-C network. A digital register coupled with logic circuitry.Iadd.including a group of serially connected shift registers.Iaddend.is then used to continuously check all received signals.Iadd.as they are received .Iaddend.to search for the unique pulsetrains transmitted by the master and secondary stations. Themicroprocessor internal to my novel LORAN-C equipment analyzes allsignals output from the .[.register.]. .Iadd.registers .Iaddend.andlogic circuitry indicating that signals from master or secondarystations have been received to first determine if they match the grouprepetition rate for the selected LORAN-C chain and then to develop ahistogram of the time of arrival of the signals from the secondarystations. Once the equipment has approximately located and is receivingthe pulse trains from the selected master and secondary stations, themicroprocessor causes other circuitry to go into a fine search mode.

In the fine search mode the microprocessor disables the equipment fromanalyzing any signals other than those received within 35 microsecondsof the approximate time of arrival of the signals from the secondarystations as determined using the histogram. The microprocessor alsoenables other equipment to analyze the phase of each pulse and to locatethe third cycle zero crossing point of each received pulse. In the eventthe third cycle zero crossing of a pulse is not located at theapproximate time indicated by the microprocessor, the analyzationcircuitry indicates to the microprocessor whether to add or subcontract10 microseconds to the approximate time of arrival and then repeats theanalyzation process. This analyzation process and shifting of theapproximate search point is repeated until the third cycle zero crossingof the desired pulse of the selected master and secondary station pulsetrains is located. Using an accurate crystal controlled clock internalto my novel equipment, the microprocessor then makes accurate timedifference of arrival measurements between the time of arrival ofsignals from the master station of the selected chain and the arrival ofthe pulse trains from the secondary stations. The equipment operatorutilizes other thumbwheel switches to indicate two secondary stations,the time difference of arrival information to be visually displayed. Theoperator of the LORAN-C equipment utilizes these two read-outs using aLORAN-C hydrographic chart to locate the physical position of thenavigation equipment on the surface of the earth.

In an alternative embodiment of my invention a front panel keyboard maybe utilized rather than thumbwheel switches and the microprocessor canbe programmed to perform other functions including, but not limited to,use as a calculator. Other possible uses are limited only by the amountof storage provided within the microprocessor or auxiliary memoryadjunct to the processor in a well known manner, and by the imaginationof the equipment designer.

The operator of my novel LORAN-C navigation receiver can quickly andeasily calibrate the receiver master oscillator, unlike prior artreceivers. To accomplish this, the operator places the equipment in acalibration mode wherein the output of the oscillator is comparedagainst the group repetition interval [GRI] information which has beenentered via the thumbwheel switches. The display is used to indicate tothe operator if the equipment is in calibration or requires a simpleadjustment by the operator.

The Applicant's novel LORAN-C navigation receiver will be betterunderstood upon a review of the detailed description given hereinafterin conjunction with the drawing in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a general block diagram of the Applicant's LORAN-C navigationreceiver;

FIG. 2 shows the shape of each pulse of the pulse trains transmitted byall LORAN-C master and secondary stations;

FIG. 3 is a graphical representation of the pulse trains transmitted bythe master and secondary stations within a LORAN-C chain.

FIG. 4 is a representation of a portion of a LORAN-C navigation chart;

FIGS. 5, 6, and 7 are detailed block diagrams of the Applicant'snavigation receiver;

FIG. 8 is a detailed block diagram of the smart shift register shown inFIG. 5; and

FIG. 9 shows the manner in which FIGS. 4, 5, and 6 should be arrangedwith respect to each other when reading the detailed description.

GENERAL DESCRIPTION

To understand the general or detailed operation of my novel LORAN-Creceiver, it is best to first understand the makeup of the signalstransmitted by LORAN-C stations and being received by my novel receiver.Representation of these signals are shown in FIGS. 2 and 3 which willnow be discussed.

All master and secondary stations transmit groups of pulses as brieflymentioned above, at a specified group repetition interval which isdefined as shown in FIG. 3. Each pulse has a 100 Khz. carrier and is ofa carefully selected shape shown in FIG. 2. For each LORAN-C chain agroup repetition interval (GRI) is selected of sufficient length so thatit contains time for transmission of the pulse .[.chains.]. .Iadd.trains.Iaddend.from the master station and each associated secondary station,plus time between the transmission of each pulse train from the masterstation so that the signals received from two or more stations withinthe chain will never overlap each other when received anywhere in theLORAN-C chain coverage area. Each station transmits one pulse.[.chain.]. .Iadd.train .Iaddend.ofbeight or nine pulses per GRI asshown in FIG. 3. The master station pulse .[.chain.]. .Iadd.train.Iaddend.consists of eight pulses, each shaped like the pulse shown inFIG. 2, with each of the eight pulses spaced exactly 1,000 microsecondsapart, and with a ninth pulse spaced exactly 2,000 microseconds afterthe eighth pulse. The pulse .[.chain.]. .Iadd.train .Iaddend.for each ofthe secondary stations X, Y and Z contains eight pulses shaped as shownin FIG. 2, and each of the eight pulses is also spaced exactly 1,000microseconds apart. The pictorial representation of the pulsestransmitted by the master station and the three secondary stations X, Yand Z associated therewith shown in FIG. 3 shows that the pulse trainsnever overlap each other and all are received within the grouprepetition interval. FIG. 3 also shows a representative time differenceof arrival of the pulse train from each of the secondary stations withrespect to the master station. These time difference of arrival figuresare designated Tx, Ty and Tz and are the time differences measured usingmy receiver.

It is to be recognized that the time difference of arrival betweenreception of the pulse train from the master station and the pulsetrains from each of the X, Y and Z secondary stations will varydepending upon the location of the LORAN-C receiving equipment with thecoverage area of a LORAN-C chain. In addition, the signal strength ofthe received signals from the master and secondary stations will alsovary depending upon the location of the receiving equipment, asrepresented by the different heights of the representative pulse linesshown in FIG. 3.

The delayed or spaced ninth pulse of each master station not onlyidentifies the pulse train as being from a master station, but the ninthpulse is also turned on and off by the Coast Guard in a "blink" code,well known in the art, to indicate particular faulty secondary stationsin a LORAN-C chain. These "blink" codes are published by the Coast Guardon the LORAN-C charts.

In World War II when the LORAN-C systems were installed, carrier phasecoding was used as a military security method, but after the war whenthe need for military security ceased, the phase coding was called askywave unscrambling aid. In skywave unscrambling the 100 Khz. carrierpulses from the master station and the secondary stations in a LORAN-Cchain are changed in phase to correct for skywave interference in amanner well known in the art. Skywaves are echoes of the transmittedpulses which are reflected back to earth from the ionosphere. Suchskywaves may arrive at the LORAN-C receiver anywhere between 35microseconds to 1,000 microseconds after the ground wave for the samepulse is received. In the 35 microsecond case, the skywave will overlapits own groundwave while in the 1,000 microsecond case the skywave willoverlap the groundwave of the succeeding pulse. In either case thereceived skywave signal has distortion in the form of fading and pulseshape changes, both of which can cause positional errors. In addition, askywave may be received at higher levels than a ground wave. To preventthe long delay skywaves from affecting time difference measurements, thephase of the 100 Khz. carrier is changed for selected pulses of a pulsetrain in accordance with a predetermined pattern. These phase codepatterns are published by the Coast Guard on the LORAN-C charts.

The exact pulse envelope shape of each of the pulses transmitted by allmaster and secondary stations is also very carefully selected to aid inmeasuring the exact time difference in arrival between a pulse trainfrom a master station and a pulse train from a secondary station as isknown to those skilled in the art. To make exact time differencemeasurement, one method the prior art teaches is superpositions matchingpulse envelopes of pulses from a master station and a selected secondarystation. Another method which I also utilize, is detection of a specificzero-crossing of the 100 Khz. carrier of the master and secondarystation pulses.

Now that the reader has an understanding of the nature of the signalstransmitted by the LORAN-C master and secondary stations and how theyare used for navigation purposes, the reader can better understand theoperation of my novel LORAN-C receiver which will now be described.

In FIG. 1 is seen a general block diagram of my novel LORAN-C navigationequipment. Filter and preamplifier 1 and antenna 2 are of a conventionaldesign of the type used in all LORAN-C receivers and is permanentlytuned to a center frequency of 100 Khz., which is the operatingfrequency of all LORAN-C transmitting stations. Filter 1 has a bandpassof 20 Kilohertz. Received signals are applied to smart shift register 3and zero crossing detector 6.

The signal input to zero crossing detector 6 is first amplitude limitedso that each .Iadd.positive half .Iaddend.cycle of each pulse isrepresented by a binary one and each negative half cycle is representedby a binary zero. The leading or positive edge of each binary oneexactly corresponds to the positive .[.slope.]. .Iadd.going zerocrossing .Iaddend.of each sine wave comprising each pulse. Thus,detector 6 is a positive zero-crossing detector. As will be described indetail further in this specification logic circuit 16 also provides aninput to zero crossing detector 6, not shown in FIG. 1, which sets a 10microsecond window only within which the leading edge of each binary 1may be detected. The end result is that only the positive zero-crossingof the third cycle of each pulse of the train pulse trains transmittedby each LORAN-C station is detected and an output provided by detector6.

It can be seen that latch 5 has inputs from zero crossing detector 6 andlogic circuit 4. Clock/counter 7 is a crystal controlled clock which isrunning continuously while my novel LORAN-C receiver is in operation.The count present in counter 7 at the moment that zero crossing detector6 indicates a third cycle positive zero crossing is stored in latch 5,the contents of which are then applied to multiplexer 8. Multiplexer 8is a time division multiplexer used to multiplex the many leads fromlogic circuit 16, latch 5, clock/counter 7, thumbwheel switches 11, 61and 62 through to microprocessor 9. The count in latch 5 indicates tomicroprocessor 9 the time at which each positive zero crossing isdetected.

Smart shift register 3 has a filter at its input causing it to receivethe output from receiver 1 within a narrower bandpass of five kilohertzcentered on the carrier frequency of 100 Khz. The signal input toregister 3 is also amplitude limited so that a pulse train of 1's and0's is produced that is input to a shift register therein which isshifted at a 100 Khz. rate. Because of the 100 Khz. shifting frequencyonly the pulse trains from LORAN-C master and secondary stations willresult in outputs from each of the individual stages of the shift.[.register.]. .Iadd.registers .Iaddend.internal to smart shift register3. Logic circuitry within register 3 is used to analyze the contents ofthe shift .[.register.]. .Iadd.registers .Iaddend.internal to register 3to first determine if the signals represent a pulse train from a LORAN-Cstation, secondly to determine if the pulse train is from a master or asecondary station, and finally to indicate the particular phase codingof the signals being received from a LORAN-C station. Logic circuit 4includes a latch and a circuit to store information from register 3indicating whether a pulse train is from a master or a secondary stationand further indicating the phase code transmitted. This informationstored within the latch of logic circuit 4 is applied to microprocessor9 via multiplexer 8 for use in processing received LORAN-C signals. Atthe same time the information is stored on the latch within logiccircuit 4 there is an output from circuit 4 enabling latch 5 to storethe count in clock/counter 7 which will indicate the time of occurrence.It should be noted that clock/counter 7 also has an input to multiplexer8 so that microprocessor 9 can keep track of a continuous running timeas indicated by recycles of counter 7.

The output of thumbwheel switches 11 are also input to multiplexer 8allowing the operator of my novel LORAN-C equipment to input the grouprepetition rate of a selected LORAN-C chain to microprocessor 9. Thegroup repetition rate is also called the Group Repetition Interval(GRI). In alternative embodiments of my invention thumbwheel switches 11may be replaced by a keyboard which can be used by the operator toaccess microprocessor 9 to do many things including perform navigationcalculations.

With the various types of information being input to microprocessor 9via multiplexer 8 from the circuits previously described, microprocessor9 determines if and when signals being received via filter 1 are fromthe master and secondary stations of the selected LORAN-C chain. Oncethe microprocessor 9 locates the signals from the selected masterstation, as determined by a match of the GRI number input thereto viathe four thumbwheel switches 11 with the difference in time arrivalbetween each pulse train transmitted by the selected master station,microprocessor 9 similarly locates the corresponding secondary stationsignals. To locate the secondary stations microprocessor 9 creates ahistogram from time of arrival information of any and all secondarystation signals which are stored in twenty bins or slots created by themicroprocessor in its own memory between the arrival of any twoconsecutive master station pulse trains. When signals from the secondarystations of the selected LORAN-C chain are located by secondary stationsignal counts appearing in the histogram bins at the same rate as theGRI of the selected LORAN-C chain, the microprocessor 9 performs a finersearch by creating histogram bins of a shorter time duration. Each ofthe histogram bins in which are stored the time of arrival counts of thesignals of the appropriate secondary stations is then subdivided bymicroprocessor 9 into one hundred smaller time slot histogram bins tomore closely determine the time of arrival of the pulse trains from thesecondary stations of the selected LORAN-C chain. Each of these smallerhistogram bins or slots stores counts corresponding to the time ofreceipt of signals received in consecutive twelve microsecond periods.In this manner, microprocessor 9 closely determines the time of arrivalof pulse trains from the master and secondary stations of the selectedLORAN-C chain within twelve microsecond periods.

Once microprocessor 9 determines the particular twelve microsecondhistogram time slots in which the secondary station signals are beingreceived, the microprocessor causes an enable timing signal which causesthe equipment to go into a fine search mode utilizing logic circuit 16to accurately find the third cycle positive zero crossing of each pulseof the selected master and secondary station pulse trains. To accomplishthis function, the approximate time of arrival of sequentially receivedpulses of the master and secondary station pulse trains are sequentiallyentered into latch 15 and the contents thereof are applied to comparator14. Comparator 14 compares the contents of latch 15 with the contents ofclock/counter 7 and upon there being a match, comparator 14 provides anoutput signal to logic circuit 16. The time entered into latch 15 isactually a time calculated to be 35 microseconds before the time ofarrival of each pulse of the pulse train from a selected secondarystation. The output from comparator 14 to logic circuit 16 is used tostore three timing signals therein which are received frommicroprocessor 9. These three timing signals represent lines which occur2.5 microseconds, 12.5 microseconds, and 30.0 microseconds after theoutput signal from comparator 14. At the end of each of these threetimed sequences, the phase coding of a received pulse is checked againstphase coding permanently stored in microprocessor 9. With the phasecoding information, microprocessor 9 is able to accurately locate thethird cycle zero crossing of each pulse of the pulse trains from themaster and secondary stations. In the event that the previouslydescribed signal characteristics immediately prior to and at fixedpoints during a pulse are not received, microprocessor 9 knows thatthere is an error in its calculated time placed in latch 15 andmicroprocessor 9 either increases or decreases the calculated time ofsubsequent pulse trains by 10 microseconds and the new calculated timefigure is placed in latch 15. Logic circuit 16 again analyzes incomingsignals at the aforementioned points. This process of adding orsubtracting 10 microseconds to the calculated time is repeated untilmicroprocessor 9 accurately locates the third positive zero crossing ofeach pulse of the pulse trains transmitted by each of the master andsecondary stations of the selected LORAN-C chain .[.;.]. .Iadd.. Themicroprocessor 9 .Iaddend.then determines if the received pulse trainsare from a master or a secondary station, and further determines theparticular skywave phase code being transmitted by each of the stations.

Once microprocessor 9 functioning with the other circuits in my LORAN-Creceiver has located and locked onto the pulse trains being transmittedby the master and secondary stations of the selected LORAN-C chain andhas made the desired time difference of arrival measurement that isrequired in LORAN-C operation, microprocessor 9 causes a visualindication to be given to the equipment operator via display 12. Theoutput information is plotted on a LORAN-C hydrographic chart in a wellknown manner to locate the physical position of the LORAN-C receiver.

DETAILED DESCRIPTION

Turning now to describe in detail the operation of my novel LORAN-Cequipment.

In FIG. 2 is seen the shape or waveform of every pulse transmitted byboth master and secondary LORAN-C stations. The waveform of this pulseis very carefully chosen to aid in the detection of the third carriercycle zero crossing in a manner well known in the art. One method knownin the art is to take the first derivative of the curve represented bythe envelope of the pulse shown in FIG. 2, and this first derivativeclearly indicates a point at 25 microseconds from the beginning of thepulse. The next zero crossing following this indication is the desiredzero crossing of the third cycle of the carrier frequency. Similar tothe prior art method just described, my novel LORAN-C receiver detectsthe third zero crossing for each pulse of the master station and eachsecondary station. The precise time difference of arrival measurementsto be made utilizing a LORAN-C receiver are made by measuring from thethird cycle zero crossing of the fifth pulse of the master station pulsetrain and the third carrier cycle zero crossing of the fifth pulse ofthe manually selected secondary station.

In FIG. 3 is shown a representation of the nine pulse and eight pulsesignals transmitted by a master station and the secondary stations of aLORAN-C chain. The small vertical lines each represent a pulse waveformsuch as shown in FIG. 2. The height of the vertical lines represents therelative signal strength of the pulses as received at a LORAN-Creceiver. It can be seen that the signal strength of the pulses from themaster station and each of the secondary stations are not identical.

It can be seen in FIG. 3 that the group repetition interval (GRI) isdefined as the period between the first pulses of two consecutive masterstation pulse trains for a given LORAN-C chain. This information isfound on standard LORAN-C hydrographic charts and is used to calibratethe oscillator in my novel LORAN-C receiver as will be described togreater detail further in this specification.

In a manner well known in the art, LORAN-C receiving equipment is usedto measure the time difference of arrival between the pulse train from amaster station pulse train and the pulse trains from two or moresecondary stations associated with the master station. This timedifference of arrival information is shown on FIG. 3 as T_(x), T_(y),and T_(z).

In FIG. 4 is shown a representative figure of a LORAN-C hydrographicchart. On this chart are shown three sets of arcuate curves, each set ofcurves having a five digit number thereon and suffixed by one of theletters, x, y or z. The numbers directly correspond to the timedifference of arrival information T_(x), T_(y) and T_(z) shown in FIG. 3and measured by a LORAN-C receiver. In FIG. 3 the particular secondarystation with which a set of the arcuate curves is associated isindicated by the suffix x, y, or z after the numbers on the curves.

LORAN-C charts show land masses such as island 80 on FIG. 4. For anexample, the operator of my LORAN-C receiver located on boat 81 nearisland 80 would measure the time difference of arrival informationbetween the master station and at least two of the three secondarystations in the LORAN-C chain. The operator, in making a measurementwith respect to the X secondary station would measure 379000 on myLORAN-C receiver. As can be seen in FIG. 4, the line of position (LOP)379000 is shown passing through boat 81. In a similar manner, theoperator would measure the time difference arrival information withrespect to the Y secondary station and would come up with the number699800 on the receiver. Again, the LOP for this receiver reading passesthrough boat 81. If the operator of the LORAN-C receiver measures thetime difference of arrival information with respect to the Z secondarystation the reading would show 493500 and the LOP for this reading alsopasses through boat 81. Thus, the operator can accurately fix theposition of boat 81 on the LORAN-C chart. From this position informationon the map of FIG. 4, boat 81 may, for example, be accurately navigatedtoward harbor 82 of island 80.

It will be noted that the sample LORAN-C chart shown in FIG. 4 has onlyfive digits on each LOP, but my LORAN-C receiver, has six digits. Thelowest order or sixth digit is used to interpolate between two LOPs onthe LORAN-C chart in a manner well known in the art. In the simpleexample given above, boat 81 is located exactly on three LOPs on so nointerpolation need be done to locate a LOP between those shown on thechart of FIG. 4. Thus, it should be noted that the six digit numbersobtained utilizing my equipment each included an extra zero suffixed tothe end of the five digit LOP numbers shown on the LORAN-C chart. Asixth digit other than zero on the receiver would require interpolationbetween the LOP lines on the chart.

In FIGS. 5, 6, and 7 is shown a detailed block diagram schematic of mynovel LORAN-C receiver which I will now describe in detail. FIGS. 5, 6,and 7 should be arranged as shown in FIG. 9 to best understand thedescription found hereinafter.

LORAN-C signals are received by a conventionally designed antenna 2 andconventionally designed filter and preamplifier 1, in a manner wellknown in the art. Interference caused by miscellaneous radio frequencysignals and signals from other navigational systems are essentiallyeliminated by filter 1 which utilizes filters having a 20 Khz, bandwidthcentered on 100 Khz, with a sharp drop off at either side of this band.Filter 1, being of a conventional design utilized in many LORAN-Creceivers, is not described in further detail herein. Similarly, thechoice of antenna 2 and/or the design thereof is also well known in theart and is not disclosed herein in detail for the purpose of notcluttering up the specification with details that are well known in theart and would detract from an understanding of the invention. The outputfilter 1 is undemodulated and is applied to limiter 17 in zero crossingdetector 6 and to 5 Khz, bandwidth filter 19.

When my novel LORAN-C equipment is initially placed in operation, it isin a coarse search mode wherein it is only trying to generally locatethe pulse trains from the master and secondary stations of the selectedchain. This function is accomplished by smart shift register 3 as nowdescribed. Filter 19 has a five Khz. bandwidth centered on the 100 Khz.carrier frequency of the LORAN-C signals and causes rejection of mostspurious signals. LORAN-C signals and a few spurious signals are passedthrough filter 10 to limiter 20. Limiter 20 demodulates and hard limitsthe signals input thereto so that only a chain of binary 1's is outputfrom the limiter. Each .Iadd.chain .Iaddend.of the binary 1's outputfrom limiter 20 corresponds to a spurious signal pulse or to .[.each.]..Iadd.one .Iaddend.of the pulses in the pulse trains from master andsecondary stations. These .[.pulses.]. .Iadd.binary signals .Iaddend.areapplied to smart shift register 3 which is shown in block diagram formin FIG. 5, but is shown in detail in FIG. 8 and will be described indetail further in this specification.

Smart shift register 3 is made up of ten serially connected shiftregisters, all of which are clocked or shifted at the same period as thepulses from master and secondary LORAN-C stations are received and logicgates. This is a one-thousand microsecond period as shown in FIG. 3.These ten shift registers store a window time sample of received signalswhich are analyzed to determine if the signal stored in the shiftregisters represents a pulse train from a LORAN-C master or secondarystation. Due to the clocking the sample moves in time. The logic gatesconnected to various stages of shift registers provide outputs that areused to analyze the signals temporarily stored in the register todetermine if received signals are from a master or secondary station andto determine if the received signals have what the U.S. Coast Guardrefers to as group repetition interval A or B phase coding. These phasecodes are well known to those skilled in the art. Upon smart shiftregister 3 determining that a pulse train has been received from amaster or secondary station the internal logic gates, which aredescribed in greater detail further in the specification, apply anoutput signal on one of leads MA, MB, SA, or SB, indicating if thesignals are from a master or secondary station and the particular phasecoding thereof. A signal indication that the received signals are fromeither a master of a secondary station is stored in latch 21. Inaddition, the last named signal output from register 3 is applied via ORgate 22 to the SET input of R/S flip-flop 23 to place this flip-flop inits set state with its 1 output high indicating that a pulse train fromeither a master or secondary station has been received. The 1 output ofR/S flip-flop 23 is applied via OR gate 24 to latch 5. Moreparticularly, this output signal from flip-flop 23 is applied to theclock input CK of latch 5 and causes the latch to store the contents ofBCD counter 26 in clock/counter 7 at the moment in time that it isdetermined that signals have been received from the master or secondarystation as indicated by the signal at input CK. The .[.sored.]..Iadd.stored .Iaddend.count is indicative of the real time at which thepulse train was received. As previously briefly described, the contentsstored in latch 5 are applied to multiplexer 8 in FIG. 6 to thereafterbe input to microprocessor 9.

Multiplexer 8 in FIG. 6 is required to input signals to microprocessor 9in FIG. 7 due to the limited number of input terminals to microprocessor9 and the large number of leads over which signals must be applied tothe microprocessor. Multiplexer 8 accomplishes this task utilizing timedivision multiplexing techniques. Integrated circuit multiplexers areavailable on the market, but may also be made up of a plurality of twoinput logic AND gates, one input of each of which is connected to theleads on which are the signals to be multiplexed, and the other input ofeach of which is connected to a clock and counter arrangement whichcauses ones or groups of the logic gates to have their other inputssequentially energized in a cyclic manner. In this embodiment of myinvention multiplexer 8 comprises Texas Instrument TI74151 multiplexers.

It can be seen in FIG. 6 that there are inputs to multiplexer 8 fromlogic circuit 4, latch 5, clock/counter 7, thumbwheel switches 11, 61and 62, logic circuit 16 and microprocessor 9. The signals input tomultiplexer 8 from microprocessor 9 on leads 40 are used to control theoperation of multiplexer 8.

The contents of BCD counter 26 which are stored in latch 5 in responseto the receipt of a pulse train from a master or secondary station areapplied via multiplexer 8 to microprocessor 9 and indicate to themicroprocessor the time of receipt of a valid pulse train from a masteror secondary station.

Following microprocessor 9 receiving the contents of latch 5 viamultiplexer 8, indicating the time of receipt of a pulse train from amaster or a secondary station, the microprocessor outputs a signal onLATCH RESET which is applied to reset latch 21 and clear the informationstored therein in preparation of storing a subsequent master orsecondary station indication. In addition, the CATCH RESET is appliedvia OR gate 60 to place flip-flop 23 in its reset state.

As signals being input to microprocessor 9 from latch 5 will representthe receipt of master and secondary station signals from more than oneLORAN-C station chain, microprocessor 9 requires an input from theequipment operator using thumbwheel switches 11 to indicate a particularLORAN-C chain of interest. The operator first consults a LORAN-Chydrographic chart published by the U.S. Coast Guard and finds the grouprepetition interval (GRI) for the LORAN-C station chain of interest.Using the four switches 11 the operator enters the repetition rate orGRI.

As previously described, latch 5 is used to store the count present inBCD counter 25 each time a pulse train from a master or secondarystation is detected by smart shift register 3. At the same time, theinformation stored in latch 21 is also applied to microprocessor 9 viamultiplexer 8 to indicate the signal is from a master or secondarystation and the phase coding thereof. In the previously mentionedinitial coarse search mode microprocessor 9 analyzes master andsecondary station information being input thereto via latch 5 todetermine which indication represent signals from the stations of theselected LORAN-C chain. Microprocessor 9 stores the time signalreception of the pulse trains from all master and secondary stations asindicated by the counts stored in latch 5 until it has definitelylocated and locked onto the selected stations and can thereforecalculate the time of arrival of subsequent pulse chains therefrom.

The microprocessor is programmed to create twenty bins or slots eachcorresponding to one of twenty sequential time periods of approximatelytwelve hundred microseconds duration each. The count stored in latch 5when logic circuit 4 indicates a pulse train has been received from amaster or secondary station causes a count to be stored internal tomicroprocessor 9 in the corresponding one of the twenty slots or bins.The microprocessor 9 is programmed to store the counts stored in thesetwenty bins, which make up a histogram to determine which bins containcounts indicating receipt of master and secondary station pulse trainsat the correct GRI.

Once microprocessor 9 is consistently receiving signals from the masterstation of the selected LORAN-C chain, it causes a front panel lightdesignated "M" to be lit indicating that the receiver has locked ontothe correct master station signals. As microprocessor 9 locates eachsecondary station associated with the selected LORAN-C chain, it causesa corresponding front panel light "51", "52", "53" and "54" to be lit aseach secondary station is locked onto. This indicates to the operatorwhich secondary stations are acceptable to use to make LORAN-Cmeasurements. Microprocessor 9 then takes only the ones of the twentyhistogram bins in which the selected chain master and secondary stationsignal counts are stored and subdivides each of these bins intoone-hundred bins corresponding to sequential time slots of twelvemicroseconds duration each. The process just described is repeated forthe shorter duration histogram bins created in memory internal tomicroprocessor 9 to more closely determine the time of arrival orreceipt of the pulse trains from the secondary stations of the selectedLORAN-C chain. When the above histogram processing has been accomplishedto determine the time of receipt of master and secondary station pulsetrains within twelve microseconds accuracy, microprocessor 9 generatesan enable timing signal which causes the equipment to switch from thecoarse search mode to a fine search mode to accurately make the LORAN-Ctime difference measurements as is described further in thisspecification.

To place the equipment in the fine search mode, microprocessor 9 outputsa signal on its output COARSE DISABLE. The last named signal is appliedvia OR gate 60 to the reset input R of flip-flop 23 which preventssignals from register 3 being applied to the set input S and placingflip-flop 23 in its set or one state. Microprocessor 9 also applies asignal to its FINE ENABLE output causing the equipment to go into thefine search mode wherein the time of arrival of subsequently receivedsignals is accurately made and a readout is provided on display 12.

More particularly, the FINE ENABLE signal is applied to comparator 14 inFIG. 7 to enable same. One of the two inputs to comparator 14 is theoutput from BCD counter 25 in clock 7 on lead REAL TIME. The other inputto comparator 14 is a number stored in latch 15 and this number iscalculated by microprocessor 9 as is now described. Once microprocessor9 determines the time of arrival of the signal trains from the masterand secondary stations of the selected chain in the coarse search mode,and then switches to the fine search mode, it calculates the time ofarrival of the subsequent pulse trains of the master and secondarystations from the secondary or fine (12 microsecond) histogram. Usingthe fine histogram, microprocessor 9 actually calculates a time 35microseconds prior to the expected time of arrival of a subsequentmaster or secondary pulse train and loads this information into latch 15over lead PRE-TIME under the control of another microprocessor generatedsignal on the CONTROL input. Comparator 14 compares the signal fromclock 7 with the number stored in latch 15 and upon there being a matchbetween these two digit numbers, there is an output from comparator 14which places flip-flop 30 in logic circuit 16 into its set or one state.The one output of the flip-flop 30 is connected to the reset input R ofcounter 31 and to one of the two inputs or OR gate 32. Being in its onestate the output of flip-flop 30 is high and this is applied via OR gate32 to the set input S of flip-flop 33 which is thereby placed in its setstate with its one output high.

The high one output of flip-flop 30 being supplied to reset input R ofcounter 31 causes this counter to reset to zero. Once reset to zero,counter 31 counts to a count of 8, stops counting and causes its TCoutput to go high. The TC output of counter 31 is applied to the resetinput R of counter 34 which is disabled from counting once counter 31reaches a count of eight and is thereby disabled from counting. Thisoccurs because flip-flop 30 being placed in its set state with its oneoutput high enables counter 31 to count by resetting it to zero wherebyits TC output goes to zero, thereby removing the signal to the resetinput R of counter 34. Counter 34, which is reset to zero count, isthereby enabled to count in response to the 1 MHz signal being input toits clock input CK. Counter 34 is different than counter 31 in that itcounts up to its maximum count of 10,000 and then resets itself to zeroto recount to 10,000 again and again. Because of counter 34 counting andrecounting to 10,000, its output TC has a signal thereat which occurs ata 1,000 microsecond rate due to the dividing action by counter 34 of the1 MHz signal at its CK input. Thus, counter 34 is providing outputsignals at the same rate that each of the pulses are being received inthe pulse trains from the master and secondary stations. The TC outputof counter 34 is applied to the second input of OR gate 32 and is alsoapplied to the clocking input CK of counter 31. This causes the count incounter 31 to be .[.increment.]. .Iadd.incremented .Iaddend.by one eachtime counter 34 counts to 10,000. Thus, at the end of 8,000 microsecondscounter 31 will have reached its full count and its output TC is highwhich, being applied to the reset input R of counter 34, causes counter34 to be reset to zero and to cease counting. Counter 31 will not bereset to zero until flip-flop 30 is returned to its reset state with itsone output low. This happens when output TC goes high, which beingconnected to reset input R to flip-flop 30, causes it to be reset to itszero state. This removes the high input to reset input R of counter 31,leaving the counter at its full count with its output TC high.

One of the purposes for the timing function accomplished by counters 31and 34 is to check the phase coding of the pulse trains being receivedfrom the selected master and secondary stations. Upon microprocessor 9changing over the receiver to the fine search mode, the microprocessorparallel loads the phase coding for the first eight pulses of the masterand secondary station pulse trains of the selected LORAN-C chain intoparallel/serial converter 35 of logic circuit 16. Converter 35 is aconventional shift register well-known in the art which may be loaded inparallel and then shifted out in serial to perform parallel to serialconversion. As is well known in the art, each of the pulses of the pulsetrains received from master station and secondary stations has aparticular phase coding. The phase coding is stored in microprocessor 9and is selected by information input to the equipment by the operatorusing thumbwheel switches 11. It can be seen that the clocking input CKto converter 35 is the same 1,000 microsecond signal output from counter34. Thus, the contents of converter 35 are serially shifted out at itsoutput Q at a 1,000 microsecond rate. It should be noted that the outputQ of converter 35 is connected to one of the two inputs of exclusive ORgate 36 in zero crossing detector 6. Exclusive OR gate 36 functions asan inverter in this case in a manner known to circuit designers. When aparticular one of the pulses of the pulse trains received from a masteror secondary station is of a positive phase there is no signal or a zeroon output Q from converter 35 if the phase codes match. The result isthat each radio frequency cycle of the particular pulse which is hardlimited by limiter 17 will pass directly through exclusive OR gate 36 toflip-flop 37 unchanged. Upon the expected receipt of each particularpulse of the pulse trains from the master and secondary stations whichare to be of a negative phase, converter 35 will have shifted itscontents such its output Q will be high or a one. This high inputapplied to the second input of exclusive OR gate 36 causes OR gate 36 toinvert the phase of the pulse output from limiter 17. That is, thesignal being input to detector 6 is effectively shifted 180° therebyeliminating the negative phase coding applied to the particular pulse.This is done in order that there will be an output from exclusive ORgate 36 to place flip-flop 37 in its set state at exactly the beginningof each pulse of the pulse trains from the master and secondarystations.

Fiip-flop 37 in detector 6 being placed in its set state with its oneoutput high as described heretofore, causes latch 5 to store thecontents of counter 26 at that particular moment in time. Microprocessor9 thereby receives a time indication of the beginning of each radiofrequency cycle of each of the pulses and this information is used tomake the required time difference of arrival measurements which are thebasis .[.or.]. .Iadd.of .Iaddend.the LORAN-C system. Flip-flop 37 isreturned to its reset state before the beginning of the first cycle of asubsequent pulse received from a master or secondary station by theLATCH RESET signal as described heretofore.

Microprocessor 9 determines the estimated time of arrival of the thirdcycle positive zero crossing of each of the pulses of the next to bereceived pulse train from the selected master and secondary stations.Microprocessor 9 then substracts 35 microseconds from this time whichresults in a time that should occur five microseconds before thebeginning of the first radio frequency cycle of each pulse of the masterand secondary station pulse trains. This point in time occuring 5microseconds before the beginning of each pulse of the pulse trains isoutput from microprocessor 9 on its output leads PRE-TIME and is inputto latch 15 under control of signals from the microprocessor on theinput CONTROL. The contents of latch 15 are applied to comparator 14which is enabled by the microprocessor energizing input E upon theequipment being placed in the fine search mode. It should be noted thatcomparator 14 also has an input thereto designated REAL TIME, which isthe lock output from BCD counter 26 of clock/counter 7 in FIG. 5. Uponthere being a match of the two inputs to the comparator 14, there is anoutput therefrom which places flip-flop 30 in logic circuit 16 into itsset state and its one output goes high. As mentioned heretofore, thisenables counters 31 and 34 to commence counting as previously described.The one output of flip-flop 30 is also coupled by an OR gate 32 to theset input S of flip-flop 33 to place this flip-flop in its set statewith its one output high. As seen in FIG. 6, the one output of flip-flop33 is connected to the reset inputs of counters 38, 39 and 41, and tothe clocking input CK of flip-flop 42, all in logic circuit 16. Thepurpose of these last listed circuit elements is to help microprocessor9 analyze each received pulse of the pulse trains from the master andsecondary stations to accurately determine the time of arrival of thethird cycle positive zero crossing of each pulse.

It can be seen that the clocking input CK to each of counters, 38, 39and 41 is driven by a clock signal on lead CLK. The source of thisclocking signal is the 10 megahertz clock 45 in clock/counter 7 in FIG.5. Flip-flop 33 being placed in its one state energizes the reset inputR of each of counters 38, 39 and 41, thereby resetting these counters tozero and enabling these counters to commence counting. As can be seen inFIG. 6, counter 38 is designated a 30 microsecond counter. This means itcounts and provides a signal at its output TC 30 microseconds after thiscounter is enabled to count. Similarly, counter 39 has an output signalon output TC 2.5 microseconds after this counter is enabled to count.Also, counter 41 has an output signal at output TC 12.5 microsecondsafter this counter is enabled to count. Thus, 2.5 microseconds aftercomparator 14 caused flip-flop 30 to be placed in its set state, whichthereby causes flip-flop 33 to be placed in its set state, there is anoutput from counter 39 to the clocking input CK of flip-flop 43 of logiccircuit 16. The output TC of counter 39 remains high until its resetinput R is deenergized. Similarly, 12.5 microseconds after counter 41 isenabled by resetting there is an output therefrom to the clocking inputCK of flip-flop 44. Flip-flop 43 is a D type flip-flop which will storewhatever signal is present at its D input upon its clocking input CKbeing energized. It should be noted that the D input of flip-flop 43, aswell as the D input of flip-flops 42 and 44 is obtained from the outputof exclusive OR gate 36 in zero-crossing detector 6 in FIG. 5. Theoutput of OR gate 36 is a square wave pulse corresponding to each radiofrequency cycle of each pulse of the pulse trains received from themaster and secondary LORAN-C stations and also inverted to account forphase coding as previously described.

Counter 39 will time out and cause the clocking input CK of flip-flop 43to go high at a point in time 32.5 microseconds before the expectedarrival of the third cycle positive zero crossing of each pulse. Itshould be noted that this 32.5 microsecond point occurs 2.5 microsecondsbefore the first cycle of each pulse. At that point in time only noiseshould be received by the LORAN-C equipment and, more particularly, onlynoise of a frequency that falls within the 10 kilohertz bandwidth offilter 1. Statistically noise pulses applied to the D input of flip-flop43 will occur as often as they do not occur. Thus, counter 39 energizingclocking input CK of flip-flop 43 will cause this flip-flop to storeeither zero's or one's on a proportionally equal basis if themicroprocessor 9 has accurately determined the third cycle positive zerocrossing and the output signal from counter 39 does occur prior to thebeginning of each pulse. The Q output of flip-flop 43, as well as the Qoutputs of flip-flops 42 and 44, are coupled via multiplexer 8 tomicroprocessor 9 as can be seen in FIGS. 6 and 7. Microprocessor 9receives and stores the output of flip-flop 43 for a total of 2,000samples and is programmed to average these samples received fromflip-flop 43. There will be approximately an equal number of zero's andone's received therefrom if the input to the D input of flip-flop 43 isreceived prior to any pulse of the pulse trains from the master andsecondary stations.

Counter 41 completes its count 12.5 microseconds after it is enabled bythe output signal from comparator 14 as previously described. The outputfrom counter 41 occurs 7.5 microseconds after the beginning of the firstcycle of each pulse of the pulse trains if microprocessor 9 hasaccurately determined the position of the third cycle positive zerocrossing of each pulse. This point in time will occur during themid-point of the negative cycle of the first radio frequency cycle ofeach pulse. Thus, the moment counter 41 energizes clocking input CK offlip-flop 44, the D input of this flip-flop from exclusive OR gate 36will be a zero. The result is that the Q output of flip-flop 44 willalso be a zero which will be forwarded to microprocessor 9 viamultiplexer 8 as previously described. Microprocessor 9 also stores eachoutput from flip-flop 44 for 10,000 samples, one per pulse, and isprogrammed to average these samples to determine if they arepredominantly zero representing a negative half cycle.

In the event microprocessor 9 does not initially accurately determinethe location of the third cycle positive zero crossing of each pulse ofthe pulse trains from the master and secondary stations, and this willusually happen upon microprocessor 9 initially switching the LORAN-Cequipment into its fine search mode, the outputs from flip-flops 43 and44 will not be as described immediately hereinabove. When the estimatedtime is too long, the sample points clocked into flip-flops 43 and 44 bycounters 39 and 41 respectively will both occur during each pulse of thepulse trains. As a result, the averages made by microprocessor 9 forflip-flops 43 and 44 will yield positive or negative averages and willnot yield a zero average. In response to this condition, microprocessor9 substracts 10 microseconds from the estimated time of arrival and thesequence described above is repeated. When the estimated time is tooshort the average of the stored samples at the 2.5 microsecond and 12.5microsecond points will both be zero and microprocessor 9 will add tenmicroseconds to the estimated time of arrival. This recalculation andrepeat of the circuit operation just described is repeated until theoutput from flip-flop 43 yields a zero average to microprocessor 9 andthe output from flip-flop 44 yields a negative average. Asmicroprocessor 9 gets closer to the exact time of arrival, themicroprocessor can add or substract less than 10 microseconds to thecalculated time to determine the exact estimated time of arrival figure.

Counter 38, which is also enabled to count upon receipt of the outputsignal from comparator 14 via flip-flop 33, counts to time a period of30 microseconds at the end of which it provides an output at its outputTC. Output TC from counter 38 is connected to the reset input R offlip-flop 37 in zero-crossing detector 6 and to the reset input R offlip-flop 33. Flip-flop 37 is thereby placed in its reset state with itsone output low immediately prior to the receipt of the third cyclepositive zero crossing of each received pulse of the pulse trains fromthe master and secondary stations of the selected LORAN-C chain. Thehard limited output from limiter 17 occurring immediately afterflip-flop 37 is placed in its reset state is responsive to the thirdcycle positive zero crossing of each pulse. As a result, the one outputof flip-flop 37 goes high in direct correspondence with the leading edgeof the hard limited square wave pulse output from limiter 17 andcorresponding to the third cycle position zero crossing. As previouslydescribed, this causes the count contents of BCD counter 25 to beclocked into latch 5 and indicates the exact time of receipt of thethird cycle positive zero crossing of each pulse of the pulse trains.This information is applied via multiplexer 8 to microprocessor 9 aspreviously described for processing. In response to this information,microprocessor 9 can make the desired time difference of arrivalmeasurements required in LORAN-C equipment. Upon the time difference ofarrival measurements being made, microprocessor 9 provides appropriateoutputs on its DISPLAY outputs leads which are input to display 12.

The signals output from microprocessor 9 to display 12 are applied tothe appropriate digital display units therein. Digital display unit 51is used to visually display the time difference of arrival informationfor one selected secondary station, and digital display 52 is used tovisually display the time difference of arrival information for a secondselected secondary station. The inputs of these digital displays isencoded and is appropriately decoded by anode drivers 46 and 47, anodedriver 48 and decoder/drivers 40 and 50 to drive digital displays 51 and52 respectively. These displays along with their associated decoding anddriving circuitry are well known in the art and are commerciallyavailable. In this embodiment of my invention, displays 51 and 52 areItron FG612A1 flourescent displays, but they may also be light emittingdiode displays or liquid crystal displays, or any other form of visualdisplay.

To select the secondary stations, the time difference of arrivalmeasurements for which are to be displayed on displays 51 and 52,thumbwheel switches 61 and 62 are provided. Switch 61 is physicallyadjacent to display 51 and one of the numbers "1" to "4" are selectedwith this switch to indicate to processor 9 the information to bedisplayed. Similarly, thumbwheel switch 62 is associated with display 52and is used by the equipment operator to indicate the particularsecondary station arrival measurement to be displayed on display 52.Switch 11 shows no details but is made up of right individual switchsuch as represented by switch 61 in FIG. 7. The operator of a detentedthumbwheel brings numbers into a window and output terminals of theswitch indicates the chosen number.

A signal to noise button 62 is also located on the front panel of theequipment which while depressed by the operator causes the existingdisplay on displays 51 and 52 to be replaced by a signal to noise figurefor the same secondary stations indicated by the position of thecorresponding ones of switches 61 and 62. Microprocessor 9 is programmedto calculate the signal to noise figures to be displayed and responds tothe operation of button 62 to change the display on displays 51 and 52.To make this signal to noise ratio check, microprocessor 9 storesfourteen-thousand samples of the first negative half cycle of each pulseas indicated by counter 41 described in detail hereinabove. As is easilyunderstood, pure noise would yield seven-thousand detected negative halfcycles and seven thousand positive half cycles, and a perfect signalwould yield fourteen thousand detected negative half cycles.Accordingly, numbers between seven thousand and fourteen thousandindicate the signal to noise ratio with this ratio getting higher as thecount of detected negative half cycles increases toward the samplenumber of fourteen thousand. It is numbers between seven thousand andfourteen thousand that will be displayed on displays 51 and 52 whensignal/noise button 62 on the front panel is operated.

It can readily be seen that microprocessor 9 can be programmed todisplay numbers from 0 to 100 corresponding to the range of seventhousand to fourteen thousand by using a simple interpolation algorithm.Any other number scheme may also be used to indicate signal to noise.

While that which has been described hereinabove is at present consideredto be the preferred embodiment of the invention, it is illustrativeonly, and the rapid changes in technology will make various changes andmodifications obvious to those skilled in the art without departing fromthe scope of the invention as claimed below.

Thus, for example, programming may be added to the microprocessor andthe keyboard may be used or input and the display as output to performcalculations of all kinds, or the display may, in addition, be used toprovide a digital clock with day, date and other information. In anothervariation the microprocessor may provide navigation instructions via thedisplay.

I claim:
 1. A navigation receiver-indicator providing navigationinformation by receiving and measuring differences in the time ofarrival of coded radio signals received from a plurality of navigationtransmitters, in groups of transmitters, comprising:means for selectinga group of transmitters to be used for said time difference of arrivalmeasurements, first logic means processing said radio signals as theyare received for determining when received signals are properly codedindicating they are from said navigation transmitters, and providingoutput indications of same, processor means storing and analyzing saidoutput indications from said first logic means to determine whenreceived radio signals are from transmitters of a group of transmittersselected using said selecting means, and second logic means enabled byand functioning with said processor means after said processor means hasdetermined that received radio signals are from said selected group oftransmitters to calculate the time of reception of subsequently receivedradio signals and then to analyze said last-mentioned radio signals tothereby locate a specific point of said last-mentioned radio signalsused by said processor means to accurately measure the difference intime .Iadd.of .Iaddend.arrival of said radio signals from individualtransmitters of said selected group of transmitters and provide a visualoutput of said measurements to provide navigation information.
 2. Theinvention in accordance with claim 1 wherein said processor meansreceives feedback information from said second logic means that enablessaid processor means to revise said calculated time of radio signalreception to accurately make said time difference measurements.
 3. Theinvention in accordance with claim 2 wherein a plurality of said outputindications for said selected stations are stored within said processormeans which is programmed to take an average of said output indicationsto calculate the time of reception of said radio signals subsequentlyreceived from each of said selected group of transmitting stations. 4.The invention in accordance with claim 3 wherein said feedback signalsreceived by said processor means from said second logic means are storedin said processor means which takes an average of said feedback signalsto properly determine if said calculated time of arrival of said radiosignals is correct or needs revision before making said radio signaltime difference of arrival measurements to obtain said navigationinformation.
 5. The invention in accordance with claim 4 wherein saidtransmitters are arranged in groups consisting of one mastertransmitting station and a plurality of secondary transmitting stations,wherein the radio signals transmitted by each of said master andsecondary stations comprises a series of pulses in a pulse train withthe pulse train transmitted by a master station differing from the pulsetrain transmitted by a secondary station to distinguish the two types ofstations, wherein said time difference of signal arrival measurementsare always made between the time of arrival of signals from a masterstation and selected ones of said secondary stations, and wherein saidfirst logic means comprises,a multistage shift register used for storingpulses of the pulse chains as they are received from said master andsecondary station transmitters, and third logic means connected tovarious of said stages of said shift register to analyze a pulse chainstored therein to determine if it is from a master or a secondarystation and provide an appropriate indication of said analysis to saidprocessor means.
 6. The invention in accordance with claim 5 whereinindividual pulses of said transmitted pulse trains are phase coded,phase codes for different master and secondary transmitting stationsbeing stored in said processor means, and phase code correctionapparatus is provided wherein said phase codes for the sequentiallyreceived signals from master and secondary stations of said selectedgroup of transmitters are input to said correction apparatus and used toremove the phase coding from said sequentially received signals beforeutilizing said received radio signals to make said time difference ofarrival measurements.
 7. The invention in accordance with claim 6wherein the transmitters of each group of navigation transmitters alltransmit their pulse trains at a predetermined repetitive rate peculiarto each group and wherein said selecting means is manually operable toindicate the repetitive rate of a selected group of transmitters to saidprocessor means which utilizes said rate indication to first identifysignals received from said selected group of transmitters and then tocalculate the subsequent time of reception of said radio signals fromsaid selected group .Iadd.of .Iaddend.transmitters.
 8. The invention inaccordance with claim 7 wherein said navigation receiver-indicatorfurther comprises,a display functioning with said processor means toprovide a visual display of said time difference of signal arrivalmeasurements used for navigation, and means for indicating to saidprocessor means particular ones of said secondary stations with respectto which said time difference of signal arrival measurements should bemade.
 9. The invention in accordance with claim 6 wherein said processormeans generates an enable timing signal upon calculating the time ofreception of subsequently received radio signals from a selected groupof navigation transmitters and wherein said second logic means comprisesmeans enabled by said enable timing signal for generating timing signalswhich cause samples of any received signals to be input to saidprocessor means which stores a plurality of said samples and amplitudeaverages same for analyzation to determine if said calculated time ofsignal reception is correct to locate said specific point of said radiosignals from which said time difference of signal arrival measurementsare made to derive navigation information.
 10. The invention inaccordance with claim 9 wherein said processor means generates an enabletiming signal upon calculating the time of reception of subsequentlyreceived radio signals, and wherein said timing signal generating meanscomprises:first timing means enabled by said enable timing signal toprovide a first timing signal causing a first sample of any signalsreceived by said navigation receiver to be input to said processor meanswhich stores a plurality of said first signal samples and amplitudeaverages same, said average being zero when said signal samples aretaken outside of said pulses. second timing means enabled by said enabletiming signal to provide a second timing signal causing a second sampleof any received signals to be input to said processor means which storesa plurality of said second samples and amplitudes averages same, saidsecond sample average being other than zero when said second sample istaken during receipt of any one of said pulses, and third timing meansenabled by said enable timing signal to provide a third timing signalcausing a third sample of any received signals to be input to saidprocessor means which stores a plurality of said third samples andamplitude averages same, said third sample average being other than zerowhen said third sample is taken during any one of said pulses, and fromsaid first, second and third samples said processor means determines ifsaid calculated time of arrival of said radio signals is correct andsaid processor means revises said calculated time until predeterminedpulse parameters are located from which said specific point may belocated to make said time difference of signal arrival measurements. 11.The invention in accordance with claim 1 wherein said processor meansstores the results of the analyzation of said radio signals made by saidsecond logic means for a plurality of samples and said processor meansis programmed to derive signal to noise ratio information from storedplurality of stored samples.
 12. The invention in accordance with claim11 wherein said processor means stores said analyzation samples asbinary zeros and ones with pure noise input to said navigationreceiver-indicator resulting in an equal number of binary zeros and onesfrom said plurality of samples and pure signals from said navigationtransmitters input to said receiver-indicator resulting in all binaryones from said plurality of samples, and responsive to the number ofzeros and ones in said plurality of samples, said microprocessor meanscauses a visual output to be given indicating said signal to noiseratio.
 13. A method for deriving position information for navigationpurposes by making measurements of the time period between receipt ofperiodic signals in pulse trains from a selected first or mastertransmitting station and selected ones of a plurality of secondarytransmitting stations associated with said master station in anavigation system comprising the steps of:analyzing the makeup of eachpulse train as it is being received to determine if it is from a masteror secondary station, storing an indication of the time of receipt ofeach pulse train along with an indication whether it is from a master orsecondary station, said stored indications being used to identify saidselected master station and its associated secondary stations,calculating the time of receipt of signals subsequently received fromsaid selected master station and said secondary stations afterdetermining which periodic signals are received therefrom, analyzingeach signal received at the indicated times of arrival from saidselected master station and said secondary stations to determine if theindicated time of arrival is correct and to modify said indicated timesof arrival if necessary to locate a specific point in each of saidperiodic signals. measuring the difference in time of arrival betweenthe specific point of each of the periodic signals received from saidmaster station and each of the selected secondary stations, andproviding an output reflecting said time difference of signal arrivalmeasurement used for navigation purposes.
 14. The method of derivingposition information for navigation purposes in accordance with claim 13wherein said step of calculating the time of receipt of said signalscomprises the step of analyzing said stored indications of time ofreceipt of pulse trains by averaging the stored time indications forsaid selected master station and its associated secondary stations todetermine the average of the time of arrival of said signal pulsetrains.
 15. The method of deriving position information for navigationpurposes in accordance with claim 13 further comprising the stepofchecking phase coding of each pulse of the signal pulse trainsreceived from each of the selected master stations and its associatedsecondary stations with stored phase code information to determine ifthe received pulse train from each of the master and secondary stationsis a sky wave reflected from the ionosphere which pulse train is to bedisregarded.
 16. The method of deriving navigational positioninformation in accordance with claim 15 further comprising the stepsofremoving the phase coding from the pulses of said pulse trainsreceived from said selected master station and its associated secondarystations prior to storing the indication of time of receipt of eachpulse train in order to achieve accurate measurements of the time periodbetween receipt of signals from said selected master station and each ofsaid selected secondary stations.
 17. The method of deriving positioninformation for navigation purposes in accordance with claim 13 furthercomprising the steps ofstoring the polarity characteristic at a discretepoint on each of said periodic signals received from said selectedmaster station and from its associated secondary stations for aplurality of samples, and providing an indication of the number of timesa particular polarity occurs in said plurality of samples from each ofsaid last mentioned stations to provide an indication of the signal tonoise ratio of said signals received from said last-mentioned stations..Iadd.
 18. A navigation receiver-indicator providing navigationinformation by receiving and measuring differences in the time ofarrival of coded radio signals received from a plurality of navigationtransmitters, in groups of transmitters, each of said coded radiosignals comprising a train of discrete radio frequency pulses having aknown pulse repetition rate, the transmission of said pulse trains fromeach of said navigation transmitters in a given group being repeated ata known group repetition interval, said receiver-indicatorcomprising:means for selecting a group of transmitters to be used forsaid time difference of arrival measurements, first logic meansprocessing said radio signals as they are received for determining whenreceived signals are properly coded indicating they are from saidnavigation transmitters, said first logic means comprising shifterregister means, means for continuously shifting radio signals as theyare received through said shift register means, said shift registermeans having a sufficient length to contain a window time sample ofduration at least as long as that of one of said pulse trains from oneof said navigation transmitters, said shift register means furtherincluding a set of output terminals spaced apart by the known pulserepetition rate of the pulses in said pulse trains from said navigationtransmitters, and decoding means connected to said output terminals andproviding an output indication whenever the signals contained in saidshift register means correspond to said coded radio signals from one ofsaid navigation transmitters, processor means storing and analyzing saidoutput indications from said first logic means to determine whenreceived radio signals are from transmitters of a group of transmittersselected using said selecting means, and second logic means enabled byand functioning with said processor means after said processor means hasdetermined that received radio signals are from said selected group oftransmitters to calculate the time of reception of subsequently receivedradio signals and then to analyze said last-mentioned radio signals tothereby locate a specific point of said last-mentioned radio signalsused by said processor means to accurately measure the difference intime of arrival of said radio signals from individual transmitters ofsaid selected group of transmitters and provide a visual output of saidmeasurements to provide navigation information. .Iaddend. .Iadd.19. Theinvention in accordance with claim 18 wherein said first logic meansfurther includes limiter means for converting each pulse of said pulsetrains from said navigation transmitters as they are received tocorresponding binary digital signals, and wherein said shifting meanscontinuously shifts said binary digital signals through said shiftregister means. .Iaddend. .Iadd.20. The invention in accordance withclaim 19 wherein each of said pulse trains comprises a plurality ofcycles of a radio frequency carrier of fixed frequency, and wherein saidshifting means shifts said binary digital signals through said shiftregister means at a rate at least equal to said fixed carrier frequency..Iaddend. .Iadd.21. The invention in accordance with claim 19 whereinsaid decoding means comprises logic gate means responsive to the timecoincidence of binary digital signals on each of said output terminalsfor providing said output indication. .Iaddend. .Iadd.22. The inventionin accordance with claim 19 wherein said shift register means includes aplurality of stages connected in series, each of said stages including aplurality of binary digit storage locations and one of said outputterminals. .Iaddend. .Iadd.23. The invention in accordance with claim 18wherein the individual pulses of said pulse trains are phase coded withthe pulse phase codes being different for different ones of saidnavigation transmitters in a group, and wherein said first logic meansfurther includes means for inverting the signals on selected ones ofsaid output terminals prior to the application thereof to said decodingmeans. .Iaddend. .Iadd.24. A method for deriving position informationfor navigation purposes by making measurements of the time periodbetween receipt of periodic signals in pulse trains from a selectedfirst or master transmitting station and selected ones of a plurality ofsecondary transmitting stations associated with said master station in anavigation system, each of said pulse trains comprising a plurality ofdiscrete radio frequency pulses having a known pulse repetition rate,comprising the steps of: analyzing the makeup of each pulse train as itis being received to determine if it is from a master or secondarystation, said analyzing step including the steps of continuouslyshifting the pulses of said pulse train as they are received throughshift register means of sufficient length to contain a window timesample of duration at least as long as that of one of said pulse trainsfrom one of said secondary transmitting stations, extracting a pluralityof outputs from said shift register means at points spaced apart by theknown pulse repetition rate of the pulses in said pulse trains from saidmaster and secondary transmitting stations, and decoding said outputs toprovide an output indication whenever said pulse train contained in saidshift register means corresponds to one from said master or secondarytransmitting stations, storing an indication of the time of receipt ofeach pulse train along with an indication whether it is from a master orsecondary station, said stored indications being used to identify saidselected master station and its associated secondary stations,calculating the time of receipt of signals subsequently received fromsaid selected master station and said secondary stations afterdetermining which periodic signals are received therefrom, analyzingeach signal received at the indicated times of arrival from saidselected master station and said secondary stations to determine if theindicated time of arrival is correct and to modify said indicated timesof arrival if necessary to locate a specific point in each of saidperiodic signals, measuring the difference in time of arrival betweenthe specific point of each of the periodic signals received from saidmaster station and each of the selected secondary stations, andproviding an output reflecting said time difference of signal arrivalmeasurement used for navigation purposes. .Iaddend. .Iadd.25. The methodof deriving position information for navigation purposes in accordancewith claim 24 wherein said analyzing step further includes the step ofconverting each pulse of said pulse trains as they are received tocorresponding binary digital signals, and wherein said shifting stepcomprises the step of continuously shifting said binary digital signalsthrough said shift register means. .Iaddend. .Iadd.26. The method ofderiving position information for navigation purposes in accordance withclaim 25 wherein each pulse of said pulse trains comprises a pluralityof cycles of a radio frequency carrier of fixed frequency, and whereinsaid shifting step comprises the step of shifting said binary digitalsignals through shift register means at a rate at least equal to saidfixed carrier frequency. .Iaddend. .Iadd.27. The method of derivingposition information for navigation purposes in accordance with claim 25wherein said decoding step comprises the step of providing said outputindication in response to the time coincidence of binary digital signalson each of said outputs from said shift register means. .Iaddend..Iadd.28. The method of deriving position information for navigationpurposes in accordance with claim 24 wherein the individual pulses ofsaid pulse trains are phase coded with the pulse phase codes beingdifferent for said master and secondary stations in said system, andwherein said analyzing step further includes the step of inverting thesignals at selected ones of said outputs from said shift register meansprior to the operation of said decoding step. .Iaddend. .Iadd.29. Anavigation receiver providing navigation information by receiving andmeasuring differences in the time of arrival of radio frequency pulsetrains periodically transmitted by a plurality of navigationtransmitters in a group, the individual pulses in each of said pulsetrains having a known pulse repetition rate, said receiver comprising:means for analyzing pulse trains as they are received to determine whena given pulse train being received is from one of said transmitters,said analyzing means including shift register means, means forcontinuously shifting the pulses of said pulse trains as they arereceived through said shift register means, said shift register meanshaving a sufficient length to contain a window time sample of durationat least as long as that of one of said pulse trains from one of saidtransmitters, means for extracting a plurality of outputs from saidshift register means at points spaced apart by the known pulserepetition rate, and means for decoding said outputs to provide anoutput indication whenever the pulse train contained in said shiftregister means corresponds to one from said transmitters; and processormeans for processing said output indications from said analyzing meansto measure the difference in the time of arrival of said pulse trainsfrom individual ones of said transmitters in said group. .Iaddend..Iadd.30. The invention in accordance with claim 29 wherein saidanalyzing means further includesmeans for converting each pulse of saidpulse trains from said transmitters as they are received tocorresponding binary digital signals, and wherein said shifting meanscontinuously shifts said binary digital signals through said shiftregister means. .Iaddend. .Iadd.31. The invention in accordance withclaim 30 wherein each pulse of said pulse trains comprises a pluralityof cycles of a radio frequency carrier of fixed frequency, and whereinsaid shifting means shifts said binary digital signals through saidshift register at a rate at least equal to said fixed carrier frequency..Iaddend. .Iadd.32. The invention in accordance with claim 30 whereinsaid decoding means comprises logic gate means responsive to the timecoincidence of binary digital signals at each of said outputs from shiftregister means for providing said output indication. .Iaddend. .Iadd.33.The invention in accordance with claim 30 wherein said shift registermeans includes a plurality of stages connected in series, each of saidstages including a plurality of binary digit storage locations, andwherein said extracting means extracts an output from each of saidstages. .Iaddend. .Iadd.34. The invention in accordance with claim 29wherein the individual pulses of said pulse trains are phase coded withthe pulse phase codes being different for different ones of saidtransmitters in said group, and wherein said analyzing means furtherincludes means for inverting selected ones of said outputs from saidshift register means prior to the application thereof to said decodingmeans. .Iaddend. .Iadd.35. A method of providing navigation informationby receiving and measuring differences in the time of arrival of radiofrequency pulse trains periodically transmitted by a plurality ofnavigation transmitters in a group, the individual pulses in each ofsaid pulse trains having a known pulse repetition rate, said methodcomprising the steps of: analyzing pulse trains as they are received todetermine when a given pulse train being received is from one of saidtransmitters, said analyzing step including the steps ofcontinuouslyshifting the pulses of said pulse trains as they are received throughshift register means of sufficient length to contain a window timesample of duration at least as long as that of one of said pulse trainsfrom one of said transmitters, extracting a plurality of outputs fromsaid shift register means at points spaced apart by the known pulserepetition rate, and decoding said outputs from said shift registermeans to provide an output indication whenever said pulse traincontained in said shift register means corresponds to one from saidtransmitters; and processing said output indications to measure thedifference in the time of arrival of said pulse trains from individualones of said transmitters in said group. .Iaddend. .Iadd.36. The methodin accordance with claim 35 wherein said analyzing step further includesthe step ofconverting each pulse of said pulse trains as they arereceived to corresponding binary digital signals, and wherein saidshifting step comprises the step of continuously shifting said binarydigital signals through said shift register means. .Iaddend. .Iadd.37.The method in accordance with claim 36 wherein each pulse of said pulsetrains comprises a plurality of cycles of a radio frequency carrier offixed frequency, and wherein said shifting step comprises the step ofshifting said binary digital signals through said shift register meansat a rate at least equal to said fixed carrier frequency. .Iaddend..Iadd.38. The method in accordance with claim 36 wherein said decodingstep comprises the step of providing said output indication in responseto the time coincidence of binary digital signals at each of saidoutputs from said shift register means. .Iaddend. .Iadd.39. The methodin accordance with claim 35 wherein the individual pulses of said pulseare phase coded with the pulse phase codes being different for differentones of said transmitters in said group, and wherein said analyzing stepfurther includes the step of inverting selected ones of said outputsfrom said shift register means prior to the operation of said decodingstep..Iaddend.